The invention relates to a system for preventing fraud and shock hazard on coin telephones, and more particularly for preventing pin fraud and shock hazard from a frayed handset cord of a coin telephone.
It has become known that two types of fraud can be perpetrated on public coin telephones by fraudulently providing a low resistance connection between the handset transmitter and a grounded object, for example the frame of the coin telephone. The fraudulent connection is usually provided by driving a strong pin through the plastic surrounding the transmitter in the handset and connecting the pin to ground. In this manner local calls can be completed without depositing the initial rate coin(s), and toll calls can be completed when the ground connection to the transmitter is applied shortly before going on-hook, in which case the coin collect signal is shorted, preventing the coins from being sent to the coin box, after which the coins can be refunded to the caller.
Pin fraud causes damage to the telephone and expenses to the telephone company, which must still pay a share of the toll to the long distance carrier.
Prior art for prevention is known for example from U.S. Pat. No. 4,862,494, which shows circuitry that senses a fraudulent ground connection inside the handset. Other inventions accomplish the same object in various ways.
The prior art inventions have various drawbacks, in that they do not prevent the performance of a fraudulent call, and are difficult to implement and require additional signalling to inform the control office personnel of the fraudulent arrangement.
Another problem with the existing coin telephones is that they can cause an electric shock to a user if the handset cord has become frayed and shows bare copper conductors, which, if touched at the moment a high voltage signal is transmitted to the coin telephone, can cause a surge shock hazard to a user of the telephone.
The instant invention prevents both the possibility of perpetrating a pin fraud and also completely eliminates the shock hazard from a frayed or damaged handset cord.